The cavities of avian bones are often
lined with extensions of the bird's air sacs. Bones
containing air sacs are said to be pneumatized. When a
bird takes off for flight, the exaggerated movement of
the wings creates an air current which fills the air
sacs within its bones and makes the bird light enough
to fly. The air current created is referred to as
"flight wind". Pneumatization of the bones does not
take place until after the bird hatches.

During periods when a hen is laying
many eggs, she stores extra calcium from her diet as
bony masses within the cavities of her long
bones. The hen's body draws calcium from these
masses to make egg shells. If the hen cannot get
enough calcium in her diet to form the bony masses,
calcium will be drawn directly from her bones. This
will weaken her bones and may still produce a weak
shell. The chick embryo relies on its shell, not
only for protection, but also as a source of calcium
once its bones begin to ossify.

In humans the last five vertebrae of the spinal
column fuse to form the sacrum and the joints between
the ilia and the sacrum, the sacroiliac joints, are
movable. The sacrum makes the pelvis strong and stable
by transferring the weight of the body through the
sacroiliac joints to the pelvis and legs. In birds
fusion occurs in the embryo between the last thoracic
vertebrae, the five lumbar vertebrae, the two sacral
vertebrae, and the first five caudal vertebrae. The
resulting synsacrum is fused to the ilia. and pelvic
girdle and cannot move. This structure is a flight
adaptation in birds to reduce the shock of
landing.